Electrical power plants may operate their gas turbines continuously, except for unexpected outages or maintenance, or on a non-continuous or dispatch basis, driven by cyclic electrical demand patterns. The latter plants often have idle turbines that are used only during “peak” hours when consumer demand for electricity is high. The duration of peak hours may vary depending on many factors, including the time of year, which may impact air conditioning loads, and time of day, which may impact household appliance use. Gas turbine compressors intake large volumes of air that may contain salts and other contaminants that deposit on metal surfaces and later form aqueous corrosive species when exposed to condensation during off-line periods. When not in use, turbines located outdoors are exposed to multiple environmental factors that contribute to corrosion, such as rain, thermally-driven condensation and evaporation cycles, exposure to atmospheric oxygen, and even salt water entrained in the air at power plants located near coastlines. Even if located indoors, atmospheric moisture may condense on turbine surfaces and cause corrosion.
In the aqueous electrochemical corrosion of a metal surface, an oxidation process occurs together with a reduction process. For corrosion to occur, there must be a formation of ions and release of electrons at an anodic surface where oxidation or deterioration of the metal occurs. There is a simultaneous counter-reaction at the cathodic surface to consume the electrons generated at the anode. The anodic and cathodic reactions proceed at the same time and at equivalent rates. Corrosive attack on iron is an electrochemical reaction supported by an aqueous film or electrolyte layer on the metal surface that acts like an electrochemical circuit (e.g. similar to a car battery) in accordance with Equations (I) and (II):At the anode (oxidation): Fe(s)→Fe2++2e−  (I)At the cathode (reduction): 2H++2e−→H2(g)  (II)This reduction of hydrogen ions at a cathodic surface disturbs the balance between the acidic hydrogen (H+) ions and the alkaline hydroxyl (OH−) ions and makes the solution less acidic, or more alkaline, at the corroding interface. This affects the mechanism of oxygen reduction as in Equations (III) and (IV):(acid solutions): O2+4H++4e−→2H2O  (III)(neutral or basic solutions) O2+2H2O+4e−→4OH−  (IV)
Iron oxidizes and corrodes much more readily when dissolved oxygen is present in the aqueous film. The aqueous film may be present from a variety of sources, including, but not limited to, water washing, atmospheric condensation, rainwater, and seawater mist in coastal areas. When dissolved oxygen is present, both generalized corrosion and oxygen pitting may occur. Generalized corrosion results in a loss of metal from the entire surface. Oxygen pitting results in a highly localized loss of surface metal that may result in a large defect or stress concentration on the metal surface, leading to cracking and component failure. It is well known that certain negatively charged ions (termed anions), such as chloride (Cl−) and sulfate (SO42−), can accelerate corrosion reactions by migrating to the anodic sites and facilitating the neutralization and solvation of the new formed ferrous ions (Fe2+), which as shown above, are the first products of the iron corrosion, or oxidation, reaction at the anode. This effect is often termed “depolarization”. These anions can facilitate or accelerate both localized corrosion reactions, often termed “pitting attack”, as well as general corrosion over a wider area of the affected metal surface.